The instant invention relates to electrophotocopying machines, and more particularly to cold pressure fusing rollers used to fix toner material to copy sheets.
The process of copying indicia by the technique of forming an electroscopic powder on a surface of a substrate is well developed. Typically, this is accomplished in either of two ways. In either of these methods, a phtoconductive surface is electrically charged and then exposed to a light and shadow pattern of the image. The electrostatic charge which was deposited upon the photoconductive surface is removed from those places exposed to light, leaving a latent electrostatic image which can be developed. Development is accomplished by exposing the electrostatic image to a colored powder, usually referred to as a toner, having the property of being attracted to the electrostatic image areas. According to the first method, the toner is then fixed (permanently adhered) to the photoconductive surface (e.g. photosensitive copy paper). According to the other method, the toner image formed on the photoconductive layer is first transferred to another substrate, e.g., a sheet of ordinary paper, and is then fixed on that other substrate.
The toner is normally a blend of thermoplastic resin, carbon black or other pigment, and a small amount of dye. The exact composition depends upon the specific purpose for which it is to be used.
The fixing of the toner is normally accomplished by heat fusing the thermoplastic resin component thereof. This is usually performed by exposing the image to heat for a sufficient time to melt somewhat the toner particles so that they adhere, one to the other, and to the paper or substrate. Some fixing techniques have also employed the pressure developed by two hard surfaced rolls to fix the particles. Another process passes the substrate between hard surfaced rolls in combination with the application of heat.
Heat fusing, while producing effective results in many cases, is undesirable in that it requires that a supply of heat be available, usually entailing a supply of electric power in large quantities, and presents the possibility of overheating the substrate as well as the toner, with some danger of fire as a result. Pressure fixing as applied in the prior art has consisted of passing the toner substrate between two solid metal, hard surfaced rolls, one roll being movable by expensive spring assemblies to provide a controlled pressure. Experience has shown that obtaining uniform pressure with cold pressure fixing rollers is very difficult. Accordingly, several cold pressure fixing systems now employ rollers which are skewed (i.e. wrapped about each other) rather than parallel. However, skewed rollers present additional problems with regard to cleaning the rollers, creasing the copy paper, etc. The instant invention, therefore, overcomes the foregoing problems associated with obtaining uniform pressure with cold pressure fixing rollers by providing a pair of parallel rollers which require no springs, generate less noise, are easier to start and stop owing to less inertia to be overcome, and facilitate entry and exit of copy paper from the nip.